1. Technical Field
The present invention relates generally to ion implanter systems. In particular, the invention relates to a gas conductance restricting cathode system for use in an ion implanter system.
2. Related Art
Ion implanter systems include a component referred to as a source, which generates an ion beam. An ion beam source receives a gas from a gas source and ionizes the gas into a plasma by exposing the gas to energetic electrons. The energetic electrons may be generated in a number of ways. One conventional approach to ionize the gas has been to directly expose the gas to a heated filament in an arc chamber. The heated filament may include, for example, tungsten. One problem with this approach, however, is that the filament has a relatively small mass and, accordingly, a short lifespan due to its direct exposure to the plasma.
In order to address this problem, indirect heated cathode (IHC) sources have been implemented in which a heated filament heats a cathode that is exposed to the gas. Referring to FIG. 1, typically, an IHC source 10 includes a relatively larger mass cathode element 12 that encloses a heated filament 14 and presents the cathode into an arc chamber 16 through an aperture 18 in the arc chamber wall 20. A filament energy supply 22 provides high current electricity to heat the filament 14 such that it emits electrons. A bias voltage is applied by a bias power supply 24 between cathode element 12 and filament 14 that propels the electrons to the cathode element. The energy transfer heats cathode element 12 such that electrons are emitted from cathode element 12 in arc chamber 16. An arc energy supply 26 places a voltage between the arc chamber 16 and cathode element 12, which pulls electrons from cathode element 12 to form a plasma (not shown) as the electrons impinge upon gas molecules provided by a source gas 34.
A spacing 28 must be present between an inner periphery 30 of aperture 18 and an outer periphery 32 of cathode element 12 to maintain a voltage gap. Spacing 28, inter alia, increases the gas required to operate the system and presents a gas leakage problem. One approach to address this problem has been to plug the spacing with an insulator that extends about the cathode element. Typically, however, the insulator cannot withstand the high temperatures generated by the cathode, and consequently generates off-gasses. The off-gas is highly undesirable because of its effect on the plasma and downstream components.
In another approach, as shown in FIG. 1, spacing 28 is simply left in place and gas allowed to leak through the spacing. Gas may leak, for example, at an increased flow rate of about 0.2 standard cubic centimeters per minute (sccm) compared to a system that does not use an IHC. Increased gas leakage can lead to coating of the high voltage system components and an increase high voltage break down, glitch rate and general ion beam instability. In addition, the increased gas flow rate increases coatings of all downstream components such as bushings and ceramics. As a result, more frequent cleaning of the system is required. All of the above-described issues affect the cost of operation and reliability.
In view of the foregoing, there is a need in the art for a cathode system that addresses the problems of the related art.